Liquid insulation in an electromagnetic device, such as a power transformer, is subject to different types of voltages: AC voltages having a wide range of amplitudes and frequencies, and impulse (essentially, short-lived DC) voltages of even higher amplitude. The ability of liquid insulation to withstand the stresses imposed by electric fields of a particular voltage is often the most important property of such an insulation. This determines whether a particular liquid can be used as an insulation in a transformer (or any other electromagnetic device where high voltage is employed) of a given voltage rating. The selection of an insulation is important in that it can determine the design of all of the main elements of the device.
Typically, the highest voltage stressing the liquid insulation is the impulse (lightning) voltage. When the volume of liquid insulation is subjected to a critical dielectric stress produced by rising to its peak value impulse voltage, an insulation breakdown may occur.
To maximize the efficiency of electrical energy transmission and distribution, it is often necessary to utilize high current densities and high AC voltages inside the electromagnetic device. High currents lead to increased heat generation, while high voltage increases the level of electric stress applied to the insulation components of an electromagnetic device. Increased heat generation limits the maximum current which can be safely carried by the conductive elements of an electromagnetic device, and also increases the costs associated with the transmission, distribution, and final use of electrical energy due to the increased need for conductive materials. This also results in an overall increase in the weight and size of a given electromagnetic device. High electric stress also limits the voltage drop per unit of space inside an electromagnetic device, thereby leading to increased costs associated with the transfer of energy from generation point to the final user. Moreover, to compensate for higher stress, it is often necessary to increase the space between winding turns filled with insulation liquid, such as transformer oil, thereby further increasing transformer dimensions and cost.
Since electric current generates both heat and electric stress, it is important that the electric insulation performs two different functions continuously: (a) prevents current flow between different conductive components having different voltages; and (b) transfers heat from the windings and magnetic core to the outer walls of the device to be cooled. The dielectric properties of the liquid insulation are the most critical ones, for they are responsible for the functioning of a high voltage electromagnetic device, and dielectric strength of an insulation liquid cannot be compromised. As a result, practically all prior art liquid insulation systems have substantially high dielectric strength, low electric conductivity and high levels of purity; it is believed that this latter property is necessary for desirable dielectric strength. However, liquid insulation systems of the prior art also possess low heat conductivity, which prevents efficient heat transfer via conduction. Instead, electromagnetic devices generally employ Archimedes convection which results from the expansion of liquid insulation, e.g., transformer oil, upon heating to elevated temperatures so an Archimedes force develops which lifts the hot (and less dense) oil up and pushes the cold (more dense) oil down. Thus, thermal convection is established and heat transfer becomes possible from the windings to the outer wall of an electromagnetic device. This type of heat transfer, however, has relatively low efficiency and requires that there be provided special paths (ducts) inside the windings and the magnetic core so that the oil can flow through the hottest inner sections of the parts of the device which generate heat. Relying on heat transfer through the rather inefficient mechanism of Archimedes convection leads to an increase in the size of the device as well as an increase in cost by lowering the amount of conductive or magnetic material per unit volume. Moreover, even if the necessary precautions have been taken to ensure heat transfer, so-called hot spots can develop inside the windings/magnetic core assembly which further limits the voltage and current.
Prior art liquid insulation systems perform their functions within a limited range of current, voltage, and environmental conditions which define the power rating of an electromagnetic device. There is a need to expand these limits so that high power, i.e., higher voltage or current, can be transmitted via the device without compromising its safety and reliability, or so that the same power can be transmitted but with a smaller and less costly device. The present invention is directed to this, as well as other important ends.